Ensure buffer is alive in python-pdbtrack-unset-tracked-buffer

* lisp/progmodes/python.el (python-pdbtrack-unset-tracked-buffer):
Check if buffer `python-pdbtrack-tracked-buffer' is alive.
(Bug#38413)
1 job for master in 88 minutes and 5 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #4449
test-all

01:28:05

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1